The invention relates to two cycle engines having a cylinder liner with an exhaust bridge.
In a two cycle internal combustion engine, it is known to provide an exhaust port with two openings through the cylinder liner and cylinder wall, and with a bridge between the openings to prevent expansion of the piston rings into the exhaust port. However, when the bridge becomes heated it may expand into the cylinder which in turn interfers with the piston and causes heavy loading of the piston. One solution known in the prior art is to relieve the bridge. The present invention provides another solution where it is undesirable to relieve the bridge in a cylinder liner.
In two cycle engines with cylinder liners and exhaust bridges, another recurring problem is how to cool and lubricate the bridge. It is known in the prior art to provide a series of holes in the piston in the area where the piston runs on the exhaust bridge to help lubricate that area of the cylinder liner. However, the problem of cooling the exhaust bridge still remains.
The present invention uses fresh incoming fuel-air charge to lubricate and cool the exhaust bridge when the piston is on the downward stroke. In the preferred embodiment, a slot is cut in the outer diameter of the cylinder sleeve liner. The slot runs up the back side of the exhaust bridge of the cylinder liner. The bridge has a series of holes drilled therethrough into the cylinder. The piston has a relieved flat surface machined on its outer side wall in the area of the bridge such that on the downward power stroke of the piston the holes in the bridge are not closed off. The slot in the liner communicates with the crankcase so that when the crankcase is pressurized during the downward power stroke of the piston, fuel-air mixture is forced up the backside of the bridge and out through the holes to cool and lubricate the bridge. In an alternative, the slot is machined in the block before the liner is installed. A check valve may be used in the slot to ensure flow only in the desired direction in the slot.